Plants are constantly challenged by a plethora of disease-causing microorganisms. To counter the onslaught of infections by pathogens, plants have evolved a combination of constitutive and inducible defense responses.
Beneficial soil bacteria confer immunity against a wide range of foliar diseases by activating plant defenses, thereby reducing a plant's susceptibility to pathogen attack. Many bacterial and fungal pathogens are not restricted to infecting aerial or root tissues exclusively. As such, communication between aboveground and belowground components can confer a survival advantage and potentially prevent diseases. While considerable data exist on the occurrence of aboveground/belowground communication in the case of plant herbivory, evidence of similar phenomena in plant-pathogenic bacteria interactions is lacking.
A number of biocontrol bacteria, also known as plant growth promoting rhizobacteria (PGPR), protect plants from soil-borne pathogens by antagonistic mechanisms (Bais et al., 2004; Cavaglieri et al., 2005). Such bacteria colonizing on plant roots can also induce systemic resistance in aerial plant parts, which are spatially separated from the inducing PGPR (Ryu et al., 2004). This mechanism of induction of systemic resistance by root colonizing rhizobacteria in aerial plant parts is referred to as induced systemic resistance (ISR).
Plants use an array of metabolites to defend themselves against harmful organisms and to attract others that are beneficial. Despite progress toward understanding the symbiotic plant-microbe interactions, little headway has been made in identifying the genetic and biochemical changes responsible for the attraction of non-symbiotic rhizospheric microbes to plants. Although evidence exists for intraplant communication, to date there have been no reports demonstrating whether plants exude specific chemical signals through their roots to attract beneficial bacteria in the rhizosphere.
Furthermore, it is unknown whether shoot infection by pathogenic bacteria induces recruitment of beneficial rhizobacteria to the root surface. Links between interorganism signaling under distress conditions, especially between aboveground and belowground tissues, are poorly understood. Such signaling, although potentially complex due to the involvement of significant physical distances, may be an important and effective strategy in plant defense that has thus far been overlooked.